This invention relates to refrigeration systems and in particular to refrigeration systems used in household refrigerators. It is particularly but not solely applicable to refrigeration systems incorporating variable capacity compressors.
Vapour compression refrigeration systems utilise the large quantity of heat absorbed in a liquid refrigerant as it vaporises to extract heat from an enclosed space. This heat is subsequently released to the environment when the vapour is recondensed. The system operates in a closed cycle as shown in FIG. 1. First the refrigerant is vaporised in a heat exchanger situated inside the enclosed space to be cooled. The vapour is then compressed and transported to an external heat exchanger where the refrigerant condenses at a high pressure, releasing the previously absorbed heat to the environment. The heat exchangers are called the evaporator and condenser respectively. The liquid refrigerant is then returned to the evaporator via a flow control device A. In this case a capillary tube is used. A capillary to suction line heat exchanger B is optional and is commonly used to improve the overall efficiency of the system by increasing the enthalpy of vaporisation of the refrigerant. This effect is shown in FIG. 2 where the cycle without capillary to suction line heat exchange is shown by the cycle 1xe2x80x2-2xe2x80x2-3-5xe2x80x2-6 and that with is 1-2-3-4-5-6. In this case the heat exchanger is at or near the entrance of the capillary for clarity. The reference numerals 1 to 6 in FIG. 2 correspond to the positions 1 to 6 in FIG. 1 around the cycle. The enthalpy of vaporisation is measured by the change in enthalpy between points 5xe2x80x2 to 6 and 5 to 6 respectively. Greater separation indicates a greater change in enthalpy as the refrigerant vaporises.
The function of any flow control is two fold (1) to meter the liquid refrigerant from the liquid line into the evaporator at a rate commensurate with the rate at which vaporisation is occurring and (2) to maintain a pressure differential between the high and low pressure sides of the system in order to permit the refrigerant to vaporise under the desired low pressure in the evaporator while at the same time condensing at a high pressure in the condenser.
The capillary tube is the simplest of the refrigerant flow controls, consisting of a fixed length of small diameter tubing connected between the condenser and the evaporator. It is the device normally applied in small refrigerating systems. Because of the high frictional resistance resulting from its length and small bore and because of the throttling effect resulting from the gradual formation of vapour in the tube as the pressure of the liquid is reduced below its saturation pressure, the capillary tube acts to restrict the flow of liquid from the condenser to the evaporator and also to maintain the required operating pressure differential.
For any given tube length and bore the flow resistance of the tube is fixed, so the liquid flow rate through the tube is proportional to the pressure differential across the tube. Since the capillary tube and the compressor are in series, if the system is to perform efficiently the flow capacity of the tube must be chosen such that it matches the pumping capacity of the compressor at the system design pressures.
The system pressures are dependent on both the temperature of the environment and the enclosed space. At temperatures other than those which correspond to the design pressures, a mismatch will typically occur between the capillary and the compressor and the efficiency of the system will be less than maximum.
The efficiency of the system is also influenced by variation of the rate of heat required to be removed from the enclosed space. Variation can occur for instance because of door openings allowing warm air and environmental temperature changes. In vapour compression systems the rate of heat removal is proportional to the mass flow rate of the refrigerant. The essentially constant resistance to liquid flow of the capillary tube prevents any significant variation of flow rate under these conditions. Conventional refrigeration compressors are effectively constant pumping capacity devices. They address the need to vary flow rate by cycling on and off. By varying the cycling duty ratio they are effectively able to vary the rate of heat flow.
Cycling the compressor introduces other sources of system inefficiency. For instance the pressure differential is lost when the compressor is off and additional work is required to re-establish pressures at turn on. Also the condenser and evaporator heat exchangers are operated at less than optimum efficiency when the compressor is cycled.
Despite its limitations, its benefits which include cost and simplicity still make the capillary tube the flow control of choice in small refrigerating systems.
In order to eliminate loss of system efficiency due to cycling, variable capacity compressors have been developed. When used in conjunction with capillary tubes system efficiency gains can be obtained. However because of the fixed flow resistance the other limitations still limit efficiency.
It is therefor an object of the invention to provide a refrigeration system and/or method which will at least go some way toward overcoming the aforementioned disadvantages or which will at least provide the public with a useful choice.
In one aspect the invention consists in a refrigeration system comprising:
a compressor, a condensor, a flow control device, and an evaporator, all connected in refrigerant flow relation such that the refrigerant flows through the system to absorb heat at the evaporator, said flow control device comprising a capillary tube wherein in use refrigerant from said condensor enters said tube in a substantially liquid state and exits said tube in a mixed fluid/vapour state, there being a flash point in said tube at which said liquid begins to vaporize, and
variable sub-cooling means to provide additional forced cooling of the refrigerant at a region of or just prior to said capillary, said sub-cooling means variable to control the degree of said sub-cooling of the refrigerant, and thereby to control the position along said capillary at which the refrigerant reaches saturation pressure, to provide a flow control which is variable to match the system and conditions under which it operates.
Preferably said compressor is variable speed to provide varying flow capacities depending on the circumstance and said variable sub-cooling means are variable such that the flow control provided by said expansion valve matches said varied compressor.
Preferably said sub-cooling means comprises one or more thermoelectric elements in intimate thermal connection with said capillary.
Preferably said refrigeration system includes environment reactive means which are adapted to affect the degree of sub-cooling of said sub-cooling means in accordance with external environmental factors such as ambient temperature and humidity.
Preferably said refrigeration system includes optimisation means that in conjunction with said environment reactive means and with a said variable compressor varies the degree of sub-cooling and the operating capacity of said variable capacity compressor to optimise the efficiency of said refrigeration system having regard to external environmental factors and/or user usage patterns and/or monitored temperature characteristics within said refrigerator.
In a further aspect the invention consists in a method of refrigerating comprising passing a refrigerant through a refrigeration system including a condenser, a capillary flow control device and an evaporator connected in refrigerant flow relation to absorb heat at said evaporator and give off heat at said condenser, which method includes the steps of assessing one or more environmental or usage factors affecting the performance of said refrigeration system and sub-cooling said refrigerant at the entry to or along the length of said capillary flow control device to a degree varied according to said assessed factor or factors.
Preferably said method includes the step of varying the mass flow of refrigerant through said system in accordance with one or more said factors.
In a still further aspect the invention consists in a refrigerator incorporating a refrigeration system or method in accordance with any one of the above paragraphs.
In a yet further aspect the invention consists in a refrigeration system substantially as herein described with reference to FIGS. 3 to 7.
To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.